Method for frequency and mode stabilization of a tuneable laser

ABSTRACT

A method of frequency and mode stabilizing a tuneable laser, wherein one or more measurable magnitudes is/are measured, wherein the laser has been characterized with respect to a number of operation points, and wherein the values of the measurable magnitude or magnitudes are stored in a microprocessor or corresponding device. The invention is characterized by causing the values of one or more of the measurable magnitudes to be non-extreme values; storing the values of such non-extreme values as a quotient where the numerator is the derivative of a measurable magnitude δY with respect to a control current δI to one laser section, and where the denominator is the derivative of another measurable magnitude δZ with respect to said control current δI; and using the stored values to control the laser to a desired operation point.

The present invention relates to a method of stabilising frequency andmode of a tuneable laser.

Tuneable semiconductor lasers include several different sections throughwhich current is injected, these sections typically being three or fourin number. The wavelength, power and mode purity of the lasers can becontrolled by adjusting the current in the different sections. Modepurity implies that the laser is at an operation point, i.e. in acombination of the three or four injected drive currents that ischaracterised by the laser being distanced from a combination of thedrive currents with which mode jumps occur and where lasering is stableand sidemode suppression is high.

Special requirements are required for different applications withrespect to controlling wavelength. In the case of telecommunicationsapplications, it is necessary that the laser is able to retain itswavelength to a very high degree of accuracy and over a long period oftime, after having set the drive currents and the temperature. A typicalaccuracy is 0.01 nanometer while a typical time period is 20 years.

In order to be able to control the laser, it is necessary to map thebehaviour of the laser as a function of the different drive currents.This is necessary after manufacture but prior to using the laser.

It is also highly desirable to be able to lock the wavelength of a laserand have control over the mode in which the laser operates, so that saidlaser will operate as intended over a long period of time. By modecontrol is meant optimisation of the laser operation point in operation,either continuously or at regular intervals, so as to eliminate the riskof a mode jump to some other cavity mode. Furthermore, it would be verybeneficial if lasers could be automatically compensated for degradationin operation.

Several methods of mobilising the frequency and mode of a tuneable laserare known to the art. Several of these methods involve adjusting acurrent through a laser section so that the laser will continue to laseat the right frequency, while adjusting the currents through other lasersections while seeking a maximum or minimum in some measurable function,such as the laser output power.

Swedish Patent Specification No. 9900537-3 describes a method ofwavelength locking and mode monitoring a tuneable laser. In this method,as with other methods, the laser is controlled at an operation pointwhich lies at an extreme point on the measurable functions.

Certain functions have no usable extreme point. For instance, the outputpower of a GCSR laser may have a sawtooth configuration. Consequently,no extreme value is reached before the laser jumps to the next mode.

It is often found that the best laser operation point does not lie on ausable extreme point of the measurable functions, since such pointsoften mean that the laser operates close to a mode jump.

Consequently, it is beneficial to lock control of the laser at, forinstance, a given gradient of a curve, such as the curve of the outputpower as a function of reflector current. However, this causes a problemwhen the laser degrades, since such curves become “stretched” as thelaser ages, therewith changing the gradient of the curve.

The present invention solves the problem associated with the use ofvalues that are not the extreme values of measurable magnitudes forcontrolling a laser that degrades.

Accordingly, the present invention relates to a method of stabilisingthe frequency and mode of a tuneable laser in which one or moremeasurable magnitudes are measured, said laser having been characterisedwith respect to a number of operation points, wherein the values of themeasurable magnitude or magnitudes are stored in a microprocessor orcorresponding device, and wherein the method is characterised in thatthe values of one or more of the measurable magnitudes are caused to benon-extreme values; in that the values of such non-extreme values arestored as a quotient where the numerator is the derivative of ameasurable magnitude δY with respect to a control current δI to onelaser section, and where the denominator is the derivative of anothermeasurable magnitude δZ with respect to said control current δI; and inthat the stored values are used to control the laser to a desiredoperation point.

The invention will now be described in more detail partly with referenceto exemplifying embodiments thereof and partly with reference to theaccompanying drawings, in which

FIG. 1 is a schematic illustration of a laser;

FIG. 2 is a block diagram;

FIG. 3 is a diagram of front power as a function of reflector current ina GCSR laser; and

FIG. 4 is a diagram showing charge carrier density as a function ofcontrol current to a laser section.

FIG. 1 illustrates a laser that has four sections P1, N2, N3 and N4 on acarrier TC, where currents I1, I2, I3 and I4 are injected intorespective sections.

A laser is controlled by control variables in the form of currents thatare injected into the different sections of the laser or in the form ofthe voltage applied across different sections, and also the carriertemperature T. These variables control a number of laser internalvariables, of which the charge carrier density is the most essential.Temperature is ignored in the following, although it can be treated inthe same way as the control currents I.

Directly measurable characteristics, such as the laser output power atthe front mirror and back mirror respectively, the frequency of thelaser, and sidemode suppression are all functions of said internalvariables.

Only certain combinations of the internal variables result in desiredlaser properties, such as desired frequency, high sidemode suppression,desired output power, and sufficient distance to mode jumps.

When characterising the laser, good operation points are chosen atdifferent frequencies and these frequencies stored in the form ofdifferent currents injected into the various laser sections.

The relationship between the internal variables and the controlvariables changes as the laser degrades.

However, it is not possible to measure the internal variables.Consequently, it is proposed in accordance with the invention thatmeasurable variables are measured at the same time as secondaryvariables are formed so as to achieve correct laser control.

Secondary variables are created and used because after the laser hasdegraded the number of variables that can be measured directly is notsufficient to control the laser to operate in the same operation pointsas those established when characterising the laser.

Practically all measurable variables are functions of the internalvariables, so that when the control currents are used to control thelaser, by keeping said currents constant, the laser operation point willbe displaced as a result of degradation of the laser.

Consequently, the secondary variables shall only be a function of theinternal variables.

The present invention relates to a method of stabilising frequency andmode of a tuneable laser in which one or more measurable magnitudesis/are measured, said laser having been characterised with respect to anumber of operation points and the values of the measurable magnitude ormagnitudes having been stored in a microprocessor or correspondingdevice.

According to the invention, the values of one or more of the measurablemagnitudes are caused to be non-extreme values and are stored as aquotient where the numerator is a derivative of a measurable magnitudewith respect to a control current to a laser section and where thedenominator is another measurable magnitude with respect to said controlcurrent.

Said secondary variable or variables is/are precisely such quotients ofderivation, where such a quotient is not influenced by degradation ofthe laser and the secondary variable is a function of solely internalvariables.

By using such secondary variables, it is possible to control the laserby measuring measurable variables, calculating the secondary variable orvariables, and using a sufficient number of variables to control thelaser to an operation point that coincides with an operation point thatwas selected when characterising the laser.

The use of said quotients of derivatives is based on the understandingthat the quotient is constant despite the re-scaling of the ratiobetween charge carrier density and injected current that occurs as alaser degrades.

FIG. 2 illustrates a control circuit and devices used in characterisinga laser. In operation, the laser A is connected to a microprocessor Band to variable measuring units C. The laser A is controlled by themicroprocessor B, which controls and adjusts the control currents D tothe laser sections. These adjustments are based on measurements E fromthe measuring units C. The microprocessor B compares data E′ receivedfrom the measuring units C with predetermined values obtained whencharacterising the laser, these values being stored in themicroprocessor. The microprocessor also performs calculations of saidderivatives. Upon completion of the comparison, the microprocessor makesany adjustments necessary to the control currents to the different lasersections.

In characterising the laser, the light H emitted is detected, forinstance with respect to its power, sidemode suppression, frequency,etc., by analysing the light in a measuring instrument G that mayinclude a spectrum analyser and a wavelength meter. The measured valuesI are sent to a computer F which is programmed to select optimaloperation points, these operation points J then being transmitted to themicroprocessor B. The microprocessor B performs calculations of desiredderivative quotients and stores the same.

It is mentioned in the aforegoing that the quotient is such that thenumerator is the derivative of a measurable magnitude with respect to acontrol current to a laser section, and that the denominator is thederivative of another measurable magnitude with respect to said controlcurrent.

According to a highly preferred embodiment of the invention, the secondmeasurable magnitude is the frequency.

FIG. 3 is a diagram showing the front power of a laser as a function ofreflector current in a GCSR laser. An operation point is chosen betweenthe extreme values in the curve.

In the illustrated case, the numerator of the derivative quotient isδP/δI_(R) and the denominator is δf/δI_(R), where P is power, f isfrequency and I_(R) is reflector current. This quotient is constant whenthe curve in FIG. 3 is stretched out in the X-direction as a result ofdegradation of the laser.

Naturally, corresponding derivative quotients can be formed with othermeasurable parameters so as to eliminate the influence resulting from achange in the relationships as a result of laser degradation in the sameway.

According one preferred embodiment, the measurable magnitudes are one ormore of the magnitudes laser front power, laser back power, and laserfrequency.

It is highly preferred to cause said derivative quotients to be formedand used as control parameters in addition to using one or more controlparameters that consist of measurable magnitudes.

A Bragg laser and a GCSR laser can be considered by way of example.

A Bragg laser has only three sections, these being a gain section, phasesection and reflector section. It is easy to measure front power, backpower and transmitted frequency.

When the front power is too low, it is reasonable to increase thecurrent to the gain section. If the frequency becomes too low, it isreasonable to increase the current to both tuning sections.

Expressed simply, there are several operation points that give the sameoutput power and the same frequency. Such an operation point may liecloser to a mode jump than another operation point. Consequently, itwill not suffice to control the laser with the front power and thefrequency and further relationships will be required to this end. Onesuch further relationship that is independent of laser degradation isthe aforesaid derivative quotient.

According to the present invention, when concerned with a laser that hasthree sections, such as a Bragg laser, the control currents to the gainsection and the phase section of the laser are controlled on the basisof the values measured with respect to output power and wavelength, andthe irritative current through the reflector section of the laser andits phase section are determined on the basis of the values of saidquotients of said derivatives for the phase section or the reflectorsection.

With respect to a GCSR laser, which has four sections, it is suitable tocontrol the gain section so as to obtain a desired output power and tocontrol the phase section to obtain a desired frequency. However, thereremain two other sections to be controlled.

As will be evident from FIG. 3, the derivative of the front power withrespect to the reflector current varies significantly. It is thereforeappropriate to lock this derivative at a given value and to appropriatethis value as control data. However, the value changes with degradationof the laser. This re-scaling can be eliminated by dividing saidderivative with another derivative, such as the derivative of thewavelength with regard to the same current. This can be effected for thecoupler section. Four control parameters are obtained herewith, namelyfront power, frequency and two derivative quotients for the reflectorcurrent and the coupler current respectively. This gives four equationsin an equation system that has four unknowns.

It is thus preferred in accordance with the invention to control a laserthat has four sections, such as a GCSR laser, so that the controlcurrents to the gain section of the laser and its phase sectionrespectively are controlled on the basis of the values measured inrespect of output power and wavelength, and so that the current throughthe coupler section and reflector of said laser will be determined onthe basis of said quotients of respective derivatives.

It is thus apparent that the present invention not only eliminatesstretching of relationships due to degradation but also provides asufficient number of control variables to cause the laser to operate inthe same operation point as that chosen when characterising the laser.

A particular problem occurs when a mode change shall take place from oneoperation point in one mode to another operation point in another modein a degraded laser where various measurable magnitudes with respect tothe control currents were established when characterising the functionof the laser.

According to one preferred embodiment of the invention, this is carriedout in the following manner.

There is calculated the change in current through one or more sectionsthat has occurred as a result of degradation in order to retain a firstoperation point. The change in the derivative of a measurable magnitude,such as the frequency, with respect to the prevailing control current isalso calculated. Changes in relevant functions are then evaluated,partly with the aid of the change in control current and partly with theaid of the associated change in relevant derivatives.

This is illustrated in FIG. 4 with the aid of an example in which theratio between the tuning current I_(x) and the charge carrier densityN_(x) has changed as a result of degradation. K₀ is the original tuningcurve. The curve has degraded to K_(d) in time.

The laser first uses a charge carrier density N₁ that corresponds to thecurrent I₀₁ through the laser section concerned. As a result ofdegradation, the control circuit changes the current to I_(d1),therewith retaining the charge carrier density. A mode change is nowdesired to operation point 2 with charge carrier density N₂. Whencharacterising the laser, the current I₀₂ was required to obtain thischarge carrier density. The current I_(d2) can now be approximated byvirtue of knowing the difference between I_(d1) and I₀₁ and the changein the derivative δN/δI over time. This derivative, i.e. the chargecarrier density with respect to current, cannot be measured but can beestimated owing to the fact that the change in the derivative isre-scaled upon degradation of the laser with the same factor as otherderivatives with respect to the same current, for instance δf/δI.

The curve K_(d) can be approximated on the basis of this difference andchange. Laser control is then adjusted to the estimated operation point,whereafter adjustment is effected in accordance with the above by meansof front power, wavelength and quotients of one or more derivativesuntil the desired new operation point has been obtained.

It is evident that the present invention solves the problem mentioned inthe introduction.

Although the invention has been described with reference to a number ofexemplifying embodiments thereof, it will be understood that otherderivatives and quotients of derivatives can be formed in acorresponding manner for controlling a degraded laser to operate in adesired operation point.

It will therefore be understood that the present invention is notrestricted to the aforedescribed embodiments thereof and that variationscan be made within the scope of the following Claims.

1. A method of frequency and mode stabilising a tuneable laser, thelaser having been characterized with respect to a number of operationpoints, the method comprising measuring a plurality of values ofmeasurable characteristics of the laser; storing the measured values ofthe measurable characteristics, the values of one or more of themeasurable characteristics being non-extreme values; determining a ratioof a derivative, δY/δI, of a first measurable characteristic, Y, havinga non-extreme value with respect to a control current, I, over aderivative, δZ/δI, of a second measurable characteristic, Z, withrespect to the control current, I; and using the determined ratio tostabilize the laser to a desired operation point.
 2. A method accordingto claim 1, wherein the second measurable characteristic is frequency oflight output from the laser.
 3. A method according to claim 1, whereinthe measurable characteristics include front power, back power and lightoutput frequency.
 4. A method according to claim 1, further comprisingforming control parameters from the ratio and from values of themeasurable characteristics.
 5. A method according to claim 1, whereinthe laser has at least a gain section, a phase section and a reflectorsection, and further comprising controlling currents for the gain andphase sections based on measured values for output power from the laserand output frequency and controlling currents in the phase and reflectorsections based on the ratio of the derivatives.
 6. A method according toclaim 5, wherein the control current, I, is one of the currents for oneof the phase section and the reflector section.
 7. A method according toclaim 1, wherein the laser has at least a gain section, a phase section,a coupler section and a reflector section, and further comprisingcontrolling currents for the gain and phase sections based on measuredvalues of laser output power and frequency respectively, and controllingcurrents through the coupler and reflector sections based on the ratioof the derivatives.
 8. A method according to claim 7, wherein thecontrol current, I, is one of the currents for one of the phase section,the reflector section and the coupler section.
 9. A method according toclaim 1, further comprising measuring a current change in one or morelaser sections needed to retain a desired operating point compared to acharacterization current needed to retain the desired operation pointwhen the laser was originally characterized, and estimating a change inderivative of charge carrier density with respect to one of the controlcurrents by calculating the derivative of a measurable characteristicwith respect to the one of the control currents.
 10. A method accordingto claim 9, wherein estimating the change in derivative of chargecarrier density includes measuring a change in laser frequency withrespect to the one of the control currents.
 11. A method according toclaim 9, further comprising estimating changes in functions of differentmeasurable characteristics with respect to control currents usingchanges in the control currents and associated changes and the estimatedchange in derivative.
 12. A method according to claim 11, furthercomprising forming new functions of the different measurablecharacteristics with respect to the control currents, based on theestimated change in derivative, and using the new functions to controloperation of the laser.